1. Field of the Invention
The present invention relates to an ultrasonic transmitting and receiving apparatus and an ultrasonic transmitting and receiving method to be used for obtaining ultrasonic images by transmitting ultrasonic waves and receiving ultrasonic echoes.
2. Description of a Related Art
Conventionally, in order to obtain a two-dimensional or three-dimensional image, for example, like B-mode scanning, ultrasonic beams are transmitted one by one to scan an object to be inspected, and a two-dimensional or three-dimensional image is synthesized based on the obtained image information. However, according to such method, since time lag between frames is large, images in different times are synthesized and the synthesized image is blurred when imaging moving parts. Especially, when observing a part moving hard such as a circulatory organ, real time operation of at least 30 frames per second is required. In order to obtain ultrasonic images in real time, the imaging region within the object must be scanned at high speed, and the method of sequential scanning with a single ultrasonic beam is too late for the requirement.
As a countermeasure against such problem, a technology of transmitting and receiving plural ultrasonic beams simultaneously toward many directions from an ultrasonic transducer array in which plural ultrasonic transducers are arranged two-dimensionally is under study.
FIG. 15A schematically shows a state in which ultrasonic beams are transmitted from an ultrasonic transducer array included in an ultrasonic probe that is generally used. An ultrasonic transducer array 100 is fabricated by, for example, linearly arranging a number of ultrasonic transducers 101. As an element (ultrasonic transducer) used for transmission and reception of ultrasonic waves, an element in which electrodes are formed on both ends of a material having a piezoelectric property (piezoelectric element) that includes piezoelectric ceramic represented by PZT (Pb (lead) zirconate titanate) or a macromolecule piezoelectric element represented by PVDF (polyvinylidene difluoride) has been generally used.
Drive signal generating circuits including pulsers etc. are connected to these ultrasonic transducers 101, respectively. Applying a voltage to such ultrasonic transducer 101, the piezoelectric element expands and contracts by piezoelectric effect to generate ultrasonic waves. By driving plural ultrasonic transducers 101 at predetermined time intervals, spherical waves transmitted from the respective ultrasonic transducers 101 are synthesized and the ultrasonic beam having a focal point formed in a desired direction and a desired depth can be transmitted.
In addition, as shown in FIG. 15B, by applying two sets of timing pulses of A pulse and B pulse to one set of elements, an ultrasonic beam A and an ultrasonic beam B can be transmitted simultaneously in different directions. Note that, in the case where the A pulse and B pulse overlap, a common pulse as an addition result may be generated.
Hereinafter, thus simultaneously transmitted and received plural ultrasonic beams are referred to as “multi-beam”.
Now, in ultrasonic imaging, side lobes produced when transmitting ultrasonic beams become problematic. When an ultrasonic beam having directivity is transmitted, in spatial distribution of acoustic pressure intensity, a local maximum that occurs on the center axis in a transmitting direction is referred to as “main lobe”, and a local maximum that occurs in a direction other than that is referred to as “side lobe”. The side lobe is produced depending on the relationship between the element pitch of the ultrasonic transducers and ultrasonic frequency (such side lobe is referred to as “grating lobe”), or produced by the unwanted vibration of the ultrasonic transducers. Normally, an ultrasonic echo received by the ultrasonic transducer is subjected to signal processing as an echo that has propagated from the direction of the main lobe. On this account, in the case where the side lobe component is large or there is a strong reflector in the side lobe direction, an artifact (virtual image) is generated, and the image quality of the ultrasonic image is degraded.
In order to avoid such phenomenon, measures have been taken for suppressing the side lobes, such as improvement in the delay accuracy of the transmitted and reception beams and miniaturization of the element. However, there are limits to these techniques, and the side lobes have not been reduced to the sufficient level. Further, in the case where the ultrasonic beam is largely steered, or ultrasonic waves are subjected to multi-beam transmission, the level of the side lobe component becomes higher, and accordingly, it becomes more difficult to reduce it. Therefore, the effect on the image quality becomes a major problem.
U.S. Pat. No. 6,179,780 discloses the following technology for reducing the effect by the side lobes when performing multi-beam transmission and reception of ultrasonic waves. That is, a method of forming plural reception beams for one transmission beam, or a method of identifying transmission beams by changing frequencies of plural transmission beams or coding transmission beams by using Barker code, Golay code, etc. to relate them with received ultrasonic echoes are cited. Further, since there is a region called as “null line” where the acoustic pressure becomes generally zero between a main lobe and a side lobe, a method of performing alignment of the main lobe of another ultrasonic beam in the region, a method of simply separating the intervals of transmission beams, and a method of shifting the center frequencies of the transmission beams are also cited. Although these methods are for suppressing the occurrence of the side lobes, there is a limit to the degree that the side lobes can be reduced.
By the way, recently, as one of greatly developed technologies in the medical imaging technology field, X-ray CT (computed tomography) can be cited. The X-ray CT is a technology of applying X-rays from plural directions to the object and generating the tomogram of the object based on the X-rays transmitted through the object. However, since X-ray signals transmitted through the object from different hundreds of directions are required in order to obtain an X-ray CT image with high image quality, a complex and accurate mechanism for rotating the X-ray source and X-ray detecting unit, a huge amount of time and a high performance data processing unit for processing an enormous amount of data, etc. are indispensable. Accordingly, the apparatus becomes large scaled and expensive, and that has discouraged the widespread in general use. Further, the exposed dose of the X-ray applied to an object to be inspected has been also problematic.
In order to solve such problems, Japanese Patent Application Publication JP-A-9-161041 discloses a CT apparatus capable of obtaining high quality images at high speed even if the number of projection directions is small. In this CT apparatus, a generalized inverse matrix is stored which has been calculated in advance by performing singular value decomposition on the projection model matrix expressing in a matrix the relationship between predetermined plural different transmission paths and influence coefficients as the degree of effect on the projection value by respective pixels within a calculation space partitioned in a lattice form when a transmission ray passes through respective transmission paths, and then, the data for image display is generated by using the projection values on intersections of the X-rays projected from different directions (the partitioned calculation space in a lattice form) and the generalized inverse matrix.
However, this technique is for estimating the signals on the lattice points, where these transmission paths intersect, from the projection values in the different plural transmission paths (integration values of the transmission paths), and such technology can not be directly applied to the ultrasonic imaging for obtaining information on the object from the acoustic pressure intensity of the ultrasonic echoes reflected from the object.